For many decades lead was used as a very important galvanizing bath component in both batch and continuous galvanizing processes. Lead significantly reduces surface tension of molten zinc and increases its fluidity. This results in better wetting of the steel surface to be coated and higher drainage after withdrawal of the parts from the kettle. Lead to some degree counteracts the deleterious effect of iron in zinc on wetting behavior and reduces dross generation. Lead creates a specific crystalline pattern in the galvanized surface, so called spangles, which do not influence corrosion resistance and have only an aesthetic advantage.
But the main benefit of lead's presence in zinc is that it makes the process of galvanizing less demanding and less sensitive to many unfavorable circumstances, like insufficiently pickled, cleaned and even rusty steel surfaces, the absence of preheating and even drying when wet parts are immersed in molten zinc, and so on.
Unfortunately, lead cannot now be used for galvanizing steel parts coming into contact with drinking water, since small but significant quantities of lead from the galvanized parts dissolve in the water. Such dissolved lead may be accumulated in human and animal bodies with very deleterious results. Consequently, a number of state legislatures have passed laws which, starting in 1995, forbid the presence of lead in galvanized products which contact drinking water.
Attempts have been made to reduce lead level from conventional levels (for example, 0.5 to 1.4 wt. % in Prime Western grade zinc, ASTM B6-87), but all such attempts have resulted in bad wetting of steel parts leading to finished products with a high percentage of uncoated surfaces (black or bare spots).
Many patents describe improved formulations for top-flux and preflux solutions used in galvanizing. None of them, however, discloses galvanizing with low or no lead in batch or continuous operation.
For example, U.S. Pat. No. 3,740,275 describes a galvanizing preflux composition which does not contain ammonium chloride. The main ingredients of this formulation are zinc chloride (35 to 75%), borax or boric acid (5 to 15%), potassium aluminum fluoride (5 to 10%), sodium chloride (10 to 20%) and barium or calcium chloride (10 to 20%). The above ingredients are dissolved in water in the stated proportions to the strength of 18.degree. Be. When heated to 165.degree. C., they are said to produce good results on thin gauge steel.
The same author in U.S. Pat. No. 3,244,551 suggests top-flux compositions without ammonium chloride which comprise zinc chloride (55 to 70%), potassium aluminum fluoride (10 to 15%), potassium chloride (10 to 15%), sodium chloride, barium chloride and calcium chloride (10 to 15%).
To improve steel surface wetting by molten zinc, U.S. Pat. No. 3,030,242 proposes preflux formulations which contain zinc chloride, ammonium chloride and film-forming cellulose derivative like carboxymethyl cellulose (0.1 to 3.0% of the fluxing material by weight) or carboxyalkyl cellulose and their salts.
In U.S. Pat. No. 3,816,188 low fuming top-fluxes are described. They are said to provide superior fluxing activity and contain zinc chloride or zinc bromide, zinc phosphate or zinc phosphite, small amount of foaming agent and a chloride or bromide of sodium potassium, lithium, magnesium and/or calcium. It is mentioned that these fluxes are advantageously employed whether the bath metal is zinc, as in conventional galvanizing, or zinc alloy.
U.S. Pat. No. 2,473,579 describes foaming-type galvanizing top-fluxes, where certain organic compounds having at least one aromatic ring connected with at least two carbonyl groups are used as foaming agents. According to the invention, these fluxes remain remarkably fluid during use and help to obtain complete coating of steel parts by molten zinc.
All these prefluxes and top-fluxes were tested for low lead or lead-free galvanizing, but none of them gave good results. Rather, steel surface wetting by molten zinc was bad, and a lot of uncoated (bare) spots were obtained.